In the past, there have been attempts to use gaseous fluxes, such as formic acid and gaseous hydrazine for soldering of electronic circuit boards. Such gaseous fluxes are highly volatile at or near the ambient temperature and are introduced into the environment for the soldering process as gases. Such gaseous fluxes are difficult to contain in a wave soldering machine and are less effective than conventional liquid fluxes in cleaning oxide and promoting wetting. After the soldering operation, most of the gaseous flux is emitted as hazardous chemicals. Moreover, gaseous fluxes tend to leave corrosive residue which requires post-soldering cleaning.
There have been attempts to apply a flux to a work surface while avoiding volatile organic chemicals (VOCs) in the past. For example, U.S. Pat. No. 5,281,281 (Stefanowski, filed Feb. 5, 1993) suggests that the flux be dissolved in a water solvent that also includes a surfactant. The dissolved flux in water may be applied by wave, foaming or spraying. However, without cleaning or coating, the use of fluxes dissolved in water degrades product reliability. The addition of a cleaning procedure or coating requires additional floor space, longer production cycle time and higher costs.
U.S. Pat. No. 5,164,022 (Pine et al., filed Feb. 19, 1991) discloses the heating of a liquid flux in a chamber until the flux vaporizes which forces droplets of the flux out of holes in a reservoir and onto a work surface.
U.S. Pat. No. 5,024,856 (Hohnerlein, filed Dec. 29, 1989) discloses the application of droplets of flux produced by an ultrasonic atomizer to a work surface.
U.S. Pat. No. 4,821,948 (Fisher et al., filed Apr. 6, 1988) and U.S. Pat. No. 4,871,105 (Fisher et al., filed Feb. 1, 1989) disclose flux droplets that are injected into a gas stream directed against a circuit board to be soldered.
With the exception of U.S. Pat. No. 5,281,281, the foregoing prior art does not disclose a way of avoiding VOCs. U.S. Pat. No. 5,281,281 is limited in the amount of flux that can be applied to a work surface because the flux must be dissolved in a substantial amount of water and a surfactant also is necessary.